The present invention generally relates to methods and systems for the continuous in-line coating of bent concrete rebar products, known as xe2x80x9cspiralsxe2x80x9d or xe2x80x9cspiraled steelxe2x80x9d.
It is notoriously well known to employ steel or other metal reinforcing rods or bars known colloquially as xe2x80x9crebarxe2x80x9d to reinforce structural members formed of cementitious materials, such as concrete, so as to improve the concrete structure""s tensile strength. Although steel and other metal rebar can in fact enhance the tensile strength of the concrete structure, they are susceptible to oxidation. For example, ferrous metal rusts by the oxidation thereof to the corresponding oxides and hydroxides or iron by atmospheric oxygen in the presence of water.
Steel rebar within a concrete structure remains passive provided that the concrete remains highly alkaline. That is, since concrete is typically poured at a pH of 12 to 14 (i.e., at high alkalinity) due to the presence of hydroxides of sodium, potassium and calcium formed during the hydration of the concrete, oxidation of the steel rebar is typically not a concern in the short term. However, over time, exposure to a strong acid (such as typically occurs by virtue of chlorine ions from road salt, salt air in marine environments and/or salt-contaminated aggregate (e.g., sand) used to make the concrete) lowers the initial pH of the concrete thereby allowing the steel rebar therein to corrode, for example, by means of an electrolysis effect. When the rebar corrodes, it can expand and create internal stresses in the concrete which ultimately are revealed by cracking and, ultimately disintegration, of the concrete.
It has therefore been conventional practice to coat the rebar with a thermoset epoxy resin coating in order to minimize the rebar""s susceptibility to corrosion. The epoxy coating of rebar is not, however, without problems. For example, the epoxy coating on the rebar is highly susceptible to cracking during bending of the rebar to form so-called spiral steel rebar (that is, rebar bent into a generally round or rectangular cross-sectional xe2x80x9choopxe2x80x9d that is tied to vertical linear rebar in concrete columns).
Specifically, cracking of the epoxy coating can and does occur during bending if there exists a less than optimum state of cleanliness of the rebar resulting in an insufficient anchor profile patter of the surface of the bar to hold the coating, or if the coating thickness is uneven (i.e., to thin or too thick from optimum thickness. For these reasons, the spiral steel rebar is typically first formed into the desired geometric hoop configuration, and then subjected to a powder-coating operation whereby a shot blasting process (i.e., to create a roughened surface, or anchor profile on the steel) precedes a thermoset epoxy resin powder coating operation onto the anchor-profiled rebar surfaces.
Such batch coating of pre-formed spiraled steel however is problematic in that uneven blasting and/or coating thickness of the rebar along its interior typically ensues thereby leading to premature corrosion problems in use. That is, the nature of a reinforcing bar pre-formed into a spiral configuration of virtually any dimension causes problems during preparation and coating on the interior of the spiral shaped material. For example, the distance of the interior portions of the spiral shaped rebar material from both the blast heads and/or powder coating apparatus, as well as the inevitable masking of the interior portions of the spiral by the exterior portions thereof, typically contribute to unsatisfactory and/or uneven coatings. Thus, the epoxy coating thickness on the interior of the spiraled steel tends to be less than the exterior due to masking effects during the powder coating operation.
It would therefore be highly desirable if methods and systems were provided to allow spiraled steel rebar to be reliably and evenly epoxy-coated. It is towards fulfilling such a need that the present invention is directed.
Broadly, therefore, the present invention is embodied in methods and systems for the continuous coating and fabrication of spiraled steel rebar product for concrete structures. In especially preferred forms, the present invention includes methods and systems by which linear uncoated rebar is supplied to a polymeric (preferably, epoxy) powder-coating unit whereby a substantially uniform coating layer of a polymeric material is applied onto the uncoated rebar to form a linear coated rebar; and thereafter the linear coated rebar is bent into a spiraled steel rebar product. The spiraled steel rebar product of this invention could be fabricated in virtually any desired size. Thus, for example, the spiraled steel rebar product of the present invention may be in the form of a continuous steel rebar having between about 40 to about 50 spiral turns and weighing up to about 4000 pounds.
The bending unit employed to bend the linear coated rebar is provided with a series of bending wheels comprised of separated upstream and downstream bending wheels and a central bending wheel which is disposed between and below these upstream and downstream bending wheels. By bringing the linear coated rebar into contact with the series of bending wheels, the rebar may be bent gently into spiraled steel rebar product without damage to the polymeric surface coating. In this regard, it has been found that such gentle bending of the coated rebar may be advantageously accomplished using bending wheels which include a synthetic or natural rubber tire mounted on a rigid rotatable wheel member.
These and other aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof which follow.